The present invention generally relates to magnetic heads and more particularly to a high-sensitivity magnetic head that uses so-called GMR (giant magneto-resistive) effect.
Magnetic heads are used extensively from audio-visual apparatuses such as a tape recorder or video recorder to information processing apparatuses such as a computer. In an information processing apparatus, in particular, there is a persistent demand for recording a large amount of information signals in relation to processing of image data or audio data. In relation to this, there is a need of high-speed magnetic storage device having a very large storage capacity and hence a very large recording density. Such a large capacity magnetic storage device requires a magnetic head that is capable of performing writing and reading of information with a correspondingly high recording density.
The resolution of a magnetic head, which is a maximum recording density that the magnetic head can perform, is primarily determined by a gap width of the magnetic head and the distance from a recording medium to the gap. In an induction magnetic head in which a coil is wound around a magnetic core, a recording density of about 65 Mbits/inch.sup.2 is achieved. However, this recording density is substantially insufficient in view of the recording density of 20 Gbits/inch.sup.2 or more, which is expected to be required in future magnetic storage devices that use a very small recording dot.
In order to achieve the foregoing recording density of 20 Gbits/inch.sup.2 or more, it is necessary to provide a very high-sensitivity magnetic head that is capable of detecting a very feeble magnetic signal at a very high speed. Such a high-speed detection of feeble magnetic signals is not possible by an induction magnetic head that is based upon the principle of electromagnetic induction, in view of required resolution, sensitivity and response.
As a high-sensitivity magnetic head that is capable of detecting such very feeble magnetic signals formed by very tiny recording dots, there is proposed a magnetic head that is equipped with a magneto-resistive magnetic sensor. See for example, P. Ciureanu and Gavrila, Studies in Electrical and Electronic Engineering 39, "Magnetic Heads for Digital Recording," Chapter 7, Elsevier Publication, 1990.
FIG. 1 shows a magnetic head 10 that includes a so-called GMR (giant magneto-resistive) sensor in a cross-sectional view, wherein a GMR sensor is a magneto-resistive magnetic sensor most suitable for detecting extremely feeble magnetic signals. Further, FIGS. 2A and 2B show the construction of a GMR sensor used for the magnetic head 10.
Referring to FIG. 1, the magnetic head 10 is formed on a ceramic substrate 11 of A1.sub.2 O.sub.3 --TiC and includes a lower magnetic shield 12 formed on the substrate 11 and an upper magnetic shield 14 that is formed on the lower magnetic shield 12 with a non-magnetic insulation film 13 interposed therebetween. The upper and lower magnetic shields 12 and 14 form a read gap 15 at a front end part of the magnetic head 10, and the gap 15 includes a GMR magnetic sensor 16 therein.
On the upper magnetic shield 14, there is provided a magnetic pole 18 with a non-magnetic insulation film 17 interposed therebetween, and a write gap 19 is formed between the magnetic pole 18 and the upper magnetic shield 14 at the front end part of the magnetic head 10. It should be noted that a write coil 20 is disposed in the insulation film 17.
FIGS. 2A and 2B show the GMR magnetic sensor 16 of FIG. 1 respectively in a state in which there is no external magnetic field and in a state in which an external magnetic field H is applied.
Referring to FIGS. 2A and 2B, the GMR magnetic sensor 16 includes a non-magnetic main body 16A of a conductive non-magnetic material such as Cu or Ag and a plurality of generally flat ferromagnetic regions 16B formed in the main body 16A with a diameter of several ten nanometers and a thickness of 2-4 nm. The ferromagnetic regions 16B are separated from each other with an optimum distance for an exchange interaction. As a result of the exchange interaction, there appears a magneto-static coupling between the adjacent ferromagnetic regions 16B, and there appears an anti-parallel relationship in the direction of magnetization in the ferromagnetic regions 16B as indicated in FIG. 2A as a result of the magneto-static coupling, when there is no external magnetic field applied to the magnetic sensor 16.
When an electron current is injected in the state of FIG. 2A from an electrode A provided on the top surface of the main body 16A to the interior of the main body 16A, those electrons in the electron current and having an upward spin state experience a scattering by a ferromagnetic region 16B having a first direction of magnetization. On the other hand, those electrons in the electron flow and having a downward spin state experience also a scattering by another ferromagnetic region 16B having a second, opposite direction of magnetization. Thereby, the number of electrons reaching an electrode B also provided on the top surface of the main body 16A is decreased, and the magnetic sensor 16 exhibits a high-resistance.
When an external magnetic field H is applied to the GMR sensor 16 as indicated in FIG. 2B, on the other hand, the direction of magnetization is aligned in one direction for all the ferromagnetic regions 16B as indicated in FIG. 2B, and the magneto-static coupling between the adjacent ferromagnetic regions 16B is invalidated. In such a state, those electrons in the electron current injected from the electrode A and having one of the upward or downward spin state successfully reach the electrode B after passing through the main body 16A, although the electrons having the other spin state are scattered similarly to the case of FIG. 2A. Thus, the magnetic sensor 16 decreases the resistance thereof in response to the application of the external magnetic field H.
In such a GMR magnetic sensor 16, on the other hand, there arises a problem, when an electron current is caused to flow through the main body 16A by applying a voltage across the electrodes A and B on the main body 16A, that a part of the electrons in the electron current may travel along a short-circuit path P.sub.2 at the surface of the main body 16A rather than along a nominal path P.sub.1 that penetrates deeply into the interior of the main body 16A. See FIG. 3. When such a bypassing of the electron current occurs along the current path P.sub.2, the change of resistance of the GMR sensor 16 that is detected between the electrodes A and B, is masked by the electron current flowing along the current path P.sub.2. As a result of such a masking, the sensitivity of magnetic detection is deteriorated inevitably.
The foregoing problem of bypass current path P.sub.2 may be eliminated by providing another electrode C at the bottom of the main body 16A as indicted in FIG. 3 and detecting the resistance between the electrodes A and C. However, such an approach is unsuccessful in view of the small thickness of the main body 16A, which is at best about 50 nm. Because of the extremely small thickness of the main body 16A, the resistance across the electrodes A and C becomes substantially zero. Thereby, no reliable detection of resistance drop is possible.